When you Sort by Collections on your Home screen, don't worry about the latest book you were reading. The collection it is in will be listed first, and when you open that collection, your book will be listed at the top. When you open the book, your exact place (where you were last) will be preserved too.
And when you just put your Kindle to sleep with your latest book open where you are currently reading, the next time you start your Kindle, that page will be front and center as usual.
More on managing content in your collections from Amazon:
- Collections are stored on Amazon: When you create a collection on a device, Amazon will save your collection. It will appear in the Archived Items on other devices registered to your Amazon account. This allows you to transfer entire collections from one registered Kindle to another registered Kindle (or Kindle device)
- Books are associated with collections until removed: If you add a book to a collection, then delete the book from your device, it will remain associated with that collection in Archived Items on Amazon. You will just be removing it from that particular Kindle. If you download the book again, it will automatically reappear in the appropriate collection on your Home screen.
- Books can appear in more than one collection: You can associate a single book or other item from your library with multiple collections if you wish.
- Collections don't change device or Archive Items content: If you delete a book from a collection or delete an entire collection from your Kindle, it does not change the actual items saved on your Kindle or in your Archived Items on Amazon. When you delete a collection from your Kindle, any items in that collection will then appear on the Home screen instead of the collection.